Insulating overheads

[Al Thomason]

Hello,

I am restoring a 45' Monk trawler MV, and am to the point of putting the finish back into the overhead. I have refinished the Mahogany battens and am looking to use 1/8" door skins covered with a fabric or shelving liner set in between them. For insulation, researching has lead me to the 'poxilsddsxc' (arg, will never spell it :), but the stuff that does not attract water) foam board.

My concern is about ventilation with the plywood decking on top of the overhead. Seems to me that by tightly fitting the foam board between the framing and up against the plywood I will have eliminated air circulation. Am I asking for problems? Does anyone have any other ideas on how to do this fitting out?

Thank you
-al-

[Ken Liden]

When I built my new deckhouse I was faced with the same problem. My studs and roof beams are 3" so I had some room to work. In the spaces between the beams and studs I tacked in a 1" cedar nailing strip next to the outside skin and up against each framing member. The insulation was placed on the nailing strip and fastened with the appropriate nails. I used 1.5" polyisocyanurate insulation with foil face on each side. Installed in this manner I got a 1" air gap on the outside and 1/2" on the inside when the finish material is installed on top of the beams or studs. The foil faces are vapor barriers. I used a similar method for the hull.

This method was not taken lightly. I spent a lot of time with the insulation engineers and my concern was ventilation. In this method you create dead air spaces which ventilate naturally through the wood but the vapor barriers prevent moisture movement. The insulation prevents the transmission of heated air so that the space behind the insulation is much cooler than the inside temp. For rot to set in you must have the correct conditions of moisture and temperture. I have pulled out several pieces of my insulation over the last couple of years and checked the spaces with a moisture meter. All spaces were bone dry and the wood was at about 14% moisture content. We were live aboards at the time and that creates much more moisture than most boats would be subject to.

[ 04-06-2003, 12:15 PM: Message edited by: Ken Liden ]

[Al Thomason]

Ken,

Thank you very much, this is exactly the info and experience I was looking for. We too are looking to move aboard, so your real life experience helps.

-al-